linux-kernel - Re: [PATCH 2/7] mm: fix potential anon

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Message-ID: <20120904215347.GA6769@google.com>
Date:	Tue, 4 Sep 2012 14:53:47 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, riel@...hat.com, peterz@...radead.org,
	hughd@...gle.com, daniel.santos@...ox.com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 2/7] mm: fix potential anon_vma locking issue in
 mprotect()

On Tue, Sep 04, 2012 at 04:27:45PM +0200, Andrea Arcangeli wrote:
> Hi Michel,
> 
> On Tue, Sep 04, 2012 at 02:20:52AM -0700, Michel Lespinasse wrote:
> > This change fixes an anon_vma locking issue in the following situation:
> > - vma has no anon_vma
> > - next has an anon_vma
> > - vma is being shrunk / next is being expanded, due to an mprotect call
> > 
> > We need to take next's anon_vma lock to avoid races with rmap users
> > (such as page migration) while next is being expanded.
> > 
> > This change also removes an optimization which avoided taking anon_vma
> > lock during brk adjustments. We could probably make that optimization
> > work again, but the following anon rmap change would break it,
> > so I kept things as simple as possible here.
> 
> Agreed, definitely a bug not to take the lock whenever any
> vm_start/vm_pgoff are moved, regardless if they're the next or current
> vma. Only vm_end can be moved without taking the lock.
> 
> I'd prefer to fix it like this though:
> 
> -	if (vma->anon_vma && (importer || start != vma->vm_start)) {
> +	if ((vma->anon_vma && (importer || start != vma->vm_start) ||
> +           (adjust_next && next->anon_vma)) {

I think the minimal fix would actually be:

 	if (vma->anon_vma && (importer || start != vma->vm_start)) {
 		anon_vma = vma->anon_vma;
+	else if (next->anon_vma && adjust_next)
+		anon_vma = next->anon_vma;

I suppose if we were to consider adding this fix to the stable series,
we should probably do it in such a minimal way. I hadn't actually
considered it, because I was only thinking about this patch series,
and at patch 4/7 it becomes necessary to lock the anon_vma even if
only the vm_end side gets modified (so we'd still end up with what I
proposed in the end)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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